1. Field of the Invention
This invention relates generally to imaging systems, and more particularly to a rotating mirror drum radiometer imaging apparatus incorporating a two-sided mirror which alternately reflects a received signal from the two sides thereof to independent antennas as the mirror is rotated about a longitudinal axis extending therethrough.
2. Description of the Prior Art
Millimeter wave radiometers have been used as sensors in a variety of imaging systems. Typically, a single form of detector is used with some type of focusing element, such as a lens or a dish antenna, and a rotating or oscillating mirror and imaging system. The mirror is typically mounted at its center at some angle relative to the rotating shaft. Accordingly, only one side of the mirror is used to receive the signal and to reflect the signal therefrom.
In operation, as the rotating mirror rotates, it spins in a way that aims the view of the antenna along a path that rotates around the rotational axis of the mirror. An analogous arrangement is used by a lighthouse beacon light reflecting from a spinning mirror which aims the light at the horizon. In such applications the light beam appears to rotate around the lighthouse. With the radiometer, however, instead of sending out a signal it receives a signal from the mirror.
If the radiometer is mounted on a moving platform, such as a helicopter, with the mirror rotation axis in the direction of travel, a two-dimensional image can be obtained. One dimension of the image is formed as the mirror rotates the antenna aim at the different points along a circular arc. The second dimension of the scene is formed by the movement of the platform which causes the antenna to image a different circular arc for each rotation of the mirror.
While the above-described imaging radiometer works adequately for modest platform speeds and antenna aperture sizes, it would nevertheless be highly desirable in certain applications, such as high altitude imaging from an airborne platform, to increase the mirror rotation rate beyond that normally obtainable with heretofore developed imaging systems. For example, where the speed of the moving platform is quite high, the mirror rotation rate must be increased proportionally to ensure adequate coverage of the scene by the imaging system. Additionally, higher altitudes require that the antenna size be increased to improve the spatial resolution of the image. If the rotation rate of the mirror is increased and/or if the antenna size increases, then the apparatus needed to rotate the mirror at the necessary speed becomes exceedingly complex and/or large.
Another problem inherent in previously designed scanning imaging systems is their limitation in radar applications. Presently, scanning systems rely on rotating an entire antenna assembly to provide for sweeps of a scene. This requires rotating the entire antenna assembly on a rotating joint in a waveguide feed of the antenna assembly. With large antennas, careful balancing of the dish and feed horns is required even for moderate rotation rates. Higher rotation rates for faster screen refresh rates are even more difficult to achieve with presently developed scanning antenna systems.
Accordingly, it is a principal object of the present invention to provide a radiometer imaging apparatus which is capable of providing an even faster refresh rate and better image resolution than from heretofore developed imaging systems.
It is another object of the present invention to provide a radiometer imaging apparatus which includes a mirror capable of doubling the scene scans of any image at any given rotation rate of the mirror.
It is yet another object of the present invention to provide a radiometer imaging apparatus which provides for increased mirror rotation rates, and which enables larger antenna apertures to be employed than heretofore possible.
It is yet another object of the present invention to provide a radiometer imaging apparatus which is particularly well balanced even at relatively high rotation rates and which is particularly well suited for radar imaging applications on high speed platforms, such as helicopters and airplanes.
It is yet another object of the present invention to provide a radiometer imaging apparatus which may be constructed from widely available materials and components.